The present invention relates to a thermal transfer recording medium and particularly to a thermal transfer-image receiving sheet used in combination with a thermal transfer sheet.
Various thermal transfer recording systems are known in the art. Among others, a sublimation dye transfer recording system, where a thermal transfer sheet comprising a support, such as a polyester film, bearing thereon a thermal transfer layer containing a sublimable dye is heated by means of a heating medium, such as a thermal head or a laser beam, to form a dye image on a recording medium, is used as information recording means in various fields.
According to this system, a full-color image of an original can be reproduced by heating for a very short period of time. Further, the resultant image has high sharpness and excellent transparency, offering excellent half tone reproduction and gradation. By virtue of this nature, it is possible to form an image having a high quality comparable to that of a full-color photographic image.
However, heating during the formation of an image often causes a dye transfer film used to be expanded and contracted, resulting in occurrence of cockle (wrinkle) in the dye transfer film. Regarding the "cockle," the size, the number, the direction and the like vary depending upon the kind of the image formed. In general, in order to form a high-density image, the printing energy should be increased. Increased printing energy, however, results in increased frequency of cockle. A large difference in image density is also likely to cause cockle.
Further, the occurrence of cockle becomes particularly significant when an OHP sheet is used as an object on which an image is recorded. The reason for this is believed to reside in that, since the sensitivity of the OHP sheet is low, the printing energy should be increased when high-density printing is contemplated.
Since heat energy is not transmitted from the thermal head to the "cockled" portion, dropout occurs in the resultant image, causing the cockle, as such, to be reproduced. This renders the image with a reproduced cockle image area useless.
Although various methods have been attempted to prevent the occurrence of cockle, they could not completely prevent the occurrence of cockle during printing.
Regarding the conventional image-receiving medium, paper or a plastic film, or alternatively an image-receiving sheet with a receptive layer optionally formed on an image-recording surface is used as an image-receiving medium. Further, in order to prevent curling or improve the slip property, a back surface layer using an acrylic resin as a binder and containing an organic filler, such as an acrylic resin or TEFLON, or an inorganic filler, such as silica, as an additive is provided on the image-receiving medium in its surface where no image is to be formed.
The conventional back surface layer using an acrylic resin as the binder, however, is not always unsatisfactory in the effect of preventing curling.
Further, when several image-receiving sheets having the conventional back surface layer are put on top of one another and then fed one by one using a feed roll or the like, the back surface layer and the image-receiving surface are rubbed against each other, causing a problem of damage to the image-receiving surface.